Comparable vs. Comparator

Classes should implement the Comparable interface to control their natural ordering.
Objects that implement Comparable can be sorted by Collections.sort() and Arrays.sort() and can be used as keys in a sorted map or elements in a sorted set without the need to specify a Comparator.

« Interface » Comparable

+ compareTo(Object) : int

compareTo() compares this object with another object and returns a negative integer, zero, or a positiveinteger as this object is less than, equal to, or greater than the other object.Use Comparator to sort objects in an order other than their natural ordering.

« Interface » Comparator

+ compare(Object, Object) : int + equals(Object) : boolean

compare() compares its two arguments for order, and returns a negative integer, zero, or a positiveinteger as the first argument is less than, equal to, or greater than the second.

Examples:

1) Comparable

public class Book implements Comparable {
    String title;
    int    isbn;

    Book(String title, int isbn) {
        this.title = title;
        this.isbn  = isbn;
    }

    public String toString() {
        return title + " " + String.valueOf(isbn);
    }

    public boolean equals(Object object) {
        if (object instanceof Book) {
            Book other = (Book) object;

            return this.title.equals(other.title) && (this.isbn == other.isbn);
        } else {
            return false;
        }
    }

    public int compareTo(Object object) {
        Book other = (Book) object;

        if (this.title.equals(other.title)) {
            return this.isbn - other.isbn;
        }

        return this.title.compareTo(other.title);
    }
}

2)

import java.util.*;

public class TestCollections {
    static List list = new LinkedList();

    public static void main(String[] args) {
        list.add(new Book("Patterns", 12345));
        list.add(new Book("Patterns", 34567));
        list.add(new Book("Examples", 23456));

        testContains();
        testISBNSort();

        try {
            testNaturalSort();
        } catch (Throwable t) {
            System.out.println(t);
        }
    }

    static void testContains() {
        System.out.println("\nTESTING CONTAINS");

        // this is what we're looking for
        Book patterns = new Book("Patterns", 12345);

        // do we have it?
        System.out.println("We have it: " + list.contains(patterns));
    }

    static void testISBNSort() {
        System.out.println("\nTESTING ISBN SORT");

        // sort using a comparator
        Collections.sort(list, new Comparator() {
            public int compare(Object object1, Object object2) {
                Book book1 = (Book) object1;
                Book book2 = (Book) object2;

                return book1.isbn - book2.isbn;
            }
        });

        // print the collection
        Iterator i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }

    static void testNaturalSort() {
        System.out.println("\nTESTING NATURAL SORT");

        // sort if we're comparable
        Collections.sort(list);

        // print the collection
        Iterator i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }
}

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Use of comparable and comparator interface

In java the element in collections can be sorted by using TreeSet or TreeMap. To sort the data elements a class needs to implement Comparator or Comparable interface. Thats why all Wrapper classes like Integer,Double and String class implements Comparable interface.

A class implementing Comparable interface need to override compareTo(Object obj) method and put the logic for sorting.



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 The method returns an int value :-1,0,1

 It will return -1 : If this object is lesser than the passed object

 It will return  0 : If this object is same the passed object

 It will return 1 : If this object is greater than the passed object






Consider a class Person.



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 class Person {

  public String name;

  public String lastName;



  public Person(String name, String lastName){

     this.name=name;

     this.lastName=lastName;

  }

  public String getName(){

    return name;

  }

  public String getLastName(){

    return lastName;

  }



  public static void main(String arg[]){

        List<person> myList = new ArrayList<person>();



    myList.add(new Person("Robert","USA"));

    myList.add(new Person("Andy","UK"));

    myList.add(new Person("Harish","India"));

    for(Person person : myList){

     System.out.println("My name is "+person.getName());

    }

  }

 }

  Output is :

  My name is Robert

  My name is Andy

  My name is Harish

</person></person>






But now i want that the objects to be sorted on name basis should be retrieved in sorted order. 
Consider a class Person.



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 class Person implements Comparable{

  public String name;

  public String lastName;



  public Person(String name, String lastName){

     this.name=name;

     this.lastName=lastName;

  }

  public String getName(){

    return name;

  }

  public String getLastName(){

    return lastName;

  }



  public int compareTo(Object obj){

    Person p = (Person)obj;

    return this.name.compareTo(p.getName);

  }

  public static void main(String arg[]){

        List<person> myList = new ArrayList<person>();



    myList.add(new Person("Robert","USA"));

    myList.add(new Person("Andy","UK"));

    myList.add(new Person("Harish","India"));

    Collections.sort(myList);

    for(Person person : myList){

     System.out.println("My name is "+person.getName());

    }

  }

 }

  Output is :

  My name is Andy

  My name is Harish

  My name is Robert

</person></person>






Couple of things which needs to be taken in consideration:
1) Collections.sort() will sort only the collection having objects which implements either one of the comparing interface.
2) Collections.sort() will sort the same list.

Comparator interface is used when an extra logic is required to sort the objects. One need to override compare(Object obj1, Object obj2) method.For example you want the list of Person object to be sorted on the basis of complete name i.e "name lastName" but also on the other hand doesnt want to change the Person class default sorting implementation or Person class is a jar so so no code modification in it can be done. First create a Custom Comparator.



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 public class MyCustomComparator implements Comparator{

  public int compare(Object obj1, Object obj2){

   Person p1 =(Person) obj1;

   Person p2 =(Person) ob2;

   String p1Name = p1.getName()+ " " +p1.getLastName();

   String p2Name = p2.getName()+ " " +p2.getLastName();

   return p1Name.toCompareTo(p2Name);

  }

 }

// Changes made in main method of Person class.

  public static void main(String arg[]){

        List<person> myList = new ArrayList<person>();



    myList.add(new Person("Robert","USA"));

    myList.add(new Person("Robert","UK"));

    myList.add(new Person("Robert","India"));

    Collections.sort(myList new MyCustomComparator());

    for(Person person : myList){

     System.out.println("My name is "+person.getName() + " " + person.getLastName());

    }

  }



  OutPut:

  My name is Robert India

  My name is Robert UK

  My name is Robert USA

</person></person>






Couple of things which needs to be taken in consideration:
1) For Comparator interface you need to override method compare(obj)
2) In collections.sort() the instance of Comparator need to be passed. In this example the list is sorted according to the custom Comparator created